Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Long Wavelength Metamorphic InGaAs Detectors on GaAs Substrates I. Adesida, J. H. Jang, J. W. Bae, S. Kim Micro and Nanotechnology Laboratory Department of Electrical and Computer Engineering University of Illinois at Urbana-Champaign W. E. Hoke Raytheon RF Components Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Outline • Metamorphic Photodiodes – Metamorphic Device Technology – Double Heterojunction P-i-I-N Photodiodes • Heterostructure design • Fabrication processes – Wet etch process and dry etch process • Device performance – Monolithic Integration with HEMTs (High Electron Mobility Transistors) • Summary Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Device Technologies for Optoelectronic Receivers • Long Wavelength Photodiodes In 0.53 Ga 0.47 As Lattice Matched to InP 1.55 μ m • Electronic Devices • InGaAs/InAlAs HEMTs on InP • InGaAs/InP HBTs on InP • InGaAs/AlGaAs PHEMTs on GaAs • GaAs/AlGaAs HBTs on GaAs • SiGe HBTs on Si Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign What is Metamorphic Technology ? Opto-electronic Devices Opto-electronic Devices Metamorphic Buffer InP Substrates GaAs Substrates Metamorphic buffer : lattice-constant transformer from GaAs to InP Metamorphic In 1-x-y Al x Ga y As 2.5 PDs or HEMTs AlAs (Device Heterostructure) 2.0 In x Al 1-x As Bandgap (eV) 1.5 GaAs In InP In 1-x-y Al x Ga y As Al As 1.0 Ga In x Ga 1-x As 0.5 InAs Δ a=0.2155 Å GaAs Substrate 0.0 5.4 5.6 5.8 6.0 6.2 Lattice Constant ( ) Å J. H. Jang et. al. ECS conference ‘2002 (invited paper) Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Metamorphic Growth by Molecular Beam Epitaxy AlInAs Cross-Sectional TEM of 53% InGaAs Device Heterostructure AlInAs Buffer PDs or HEMTs In 0.52 Al 0.48 As (Device Heterostructure) Buffer Linearly Graded InGaAlAs In Al As M-Buffer Ga (1.5 μ m-thick) GaAs Substrate GaAs Substrate W. E. Hoke, J. H. Jang et. al. GaAs Mantech ‘2001 (invited paper) W. E. Hoke, J. H. Jang et. al. JVST B. July 2001 pp. 1505-1508 Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Advantages of Metamorphic Devices • Starting Material Costs for 4-inch Substrates InP : $1350 /cm 2 GaAs : $370 / cm 2 • Large Wafer Sizes InP : 4-inch Production Low Cost!! GaAs : 6-inch Production Manufacturing • Mechanical Robustness Easier Handling/Better yield • Mature Processing Technology for GaAs e.g. Back-side Via Etching for MMIC Process Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Double Heterojunction Photodiodes p P p i i i n N N Homojunction Single heterojunction Double heterojunction Advantages of DH Photodiodes • High Quantum Efficiency Reduced Carrier Loss due to Surface Recombination • Potential for High Bandwidth No Diffusion Current at the Anode or Cathode Drift Current dominates the Carrier Transport Potential Problems of DH Photodiodes • Parasitic Series Resistance at the Heterointerface Bandwidth Degradation • Carrier Accumulation at the Heterointerface Space Charge Field Electric Field Screening Effect Bandwidth Degradation Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Layer Structure of Metamorphic DH Photodiodes Transparent Highly doped cap anode Quaternary p + InGaAs Be : 4×10 19 cm -3 matching layer P + InAlAs Be : 5×10 18 cm -3 P + InGaAlAs Be : 5×10 18 cm -3 Photo- i InGaAs Undoped Chirped superlattice absorption InAlAs/InGaAs Undoped graded bandgap layer I InAlAs Undoped N + InAlAs Si : 5×10 18 cm -3 Transparent Linearly Graded InGaAlAs drift region S. I. GaAs Substrate Transparent Linearly graded cathode Metamorphic buffer layer J. H. Jang et. al. Electronics Lett. May 2001, pp.707-708 Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Large Bandgap Drift Region Electron Transit Time << Hole Transit Time Transit-Time Limited Bandwidth : Determined by Hole Transit Time Anode 20 Drift Velocity (10 6 cm/s) InGaAs 15 Absorption Electron 10 InAlAs Drift Layer Hole 5 Cathode 0 20 40 60 80 100 Electric Field (kV/cm) Adding a Drift Layer at the Cathode Side Reducing Diode Capacitance w/o Sacrificing Transit-Time Limited Bandwidth Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Energy Band Diagram InGaAlAs Quaternary Matching Layer • = + + 2 ( ) 0 . 76 0 . 49 0 . 20 ( ) ( ) E x x x for In Al As In Ga As − 0 . 52 0 . 48 0 . 53 0 . 47 1 g x x hc = = λ = = μ For 0.34, ( 0 . 34 ) 0 . 95 , 1 . 31 x E eV m g g E g 2 p + InGaAs 1 P + InAlAs 2 P + InGaAlAs 1 2 3 3 0 Electron Energy (eV) i-InGaAs 4 SL-GBL 5 I-InAlAs 6 4 -2 N + -InAlAs 7 Chirped Superlattice -3.5 InAlAs/InGaAs -4 -4.0 5 Graded Bandgap Layer -4.5 6 -6 7 (SL-GBL) -5.0 -5.5 0.97 0.98 0.99 1.00 1.01 -8 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Distance from Top Surface ( μ m) Only 0.1 eV Potential Barrier at InGaAs/InAlAs Heterojunction J. H. Jang et. al. J. Lightwave Technol. 2002 pp. 507-514 Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Fabrication Procedure (Wet etch process) Ti/Pt/Au (Unalloyed) P-Ohmic contact Contact resistance < 0.2 Ω -mm Isolation & Low junction Mesa & Undercut etching capacitance AuGe/Ni/Au (Alloyed) N-Ohmic contact Contact resistance < 0.2 Ω -mm Passivation : Polyimide Low dark current SiN x completes AR coating AR coating : SiN x on InAlAs/InGaAlAs Air-Bridge : 1.8 μ m Au Reduce parasitic capacitances Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Fabrication Procedure (Dry etch process) SiO 2 mask P-mesa etching Cl 2 /Ar/H 2 , ICP-RIE SiO 2 mask N-mesa etching Cl 2 /N 2 , ICP-RIE AuGe/Ni/Au (Alloyed) N-Ohmic contact Contact resistance < 0.2 Ω -mm Ti/Pt/Au (Unalloyed) P-Ohmic contact Contact resistance < 0.2 Ω -mm Low dark current Passivation : Polyimide SiN x completes AR coating AR coating : SiN x on InAlAs/InGaAlAs Air-Bridge : 1.8 μ m Au Reduce parasitic capacitances Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Fabrication (Wet Etch Process) P-mesa & undercut etching Ti/Pt/Au p-Ohmic Metal P + InAlAs/InGaAlAs i-InGaAs (Absorption) I-InAlAs (Drift) Various mixture of citric acid/H 2 O 2 for selective or non-selective etching Undercut etching of i-InGaAs layer reduce junction capacitance Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Fabrication (Dry Etch Process) P-mesa etching (InGaAs/InAlAs) Chemistry : Cl 2 /H 2 /Ar Bias voltage : -150 V Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Fabrication (Dry Etch Process) N-mesa etching (InAlAs) Chemistry : Cl 2 /N 2 Bias voltage : -150 V InP InGaAs InAlAs Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Fabricated Photodiodes SiN x AR Coated Optical Window Airbridge Polyimide Passivation Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Dark currents of various M-PDs and LM-PDs 10 -6 Step Graded SG 10 -7 Dark Current (A) SG M-buffer 10 -8 Linearly Graded LG LG M-buffer 10 -9 Lattice Matched LM LM On InP 10 -10 10 2 10 3 10 4 Area of Photodiodes ( μ m 2 ) M-PDs : Metamorphic Photodiodes on GaAs, LM-PDs : Lattice Matched Photodiodes on InP J. H. Jang et. al. J. Lightwave Technol. 2002 pp. 507-514 Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign Dark Current Characteristics High Quality Buffer & Fabrication Process •Smooth Curve No Charge Accumulation Effect Chirped Superlattice Graded Bandgap Layer, and InGaAlAs Matching layer •Effect of Passivation Technique on Dark Current of Photodiodes 10 -7 Passivated with SiN x Dark current (A) 10 -8 Unpassivated Passivated with Polyimide 10 -9 10 2 10 3 10 4 Area of devices ( μ m 2 ) J. H. Jang et. al. J. Lightwave Technol. 2002 pp. 507-514 Advanced Processing and Circuits Group
Micro and Nanotechnology Lab University of Illinois at Urbana-Champaign RF Characteristics of Photodiodes P-i-I-N Photodiode with 10 μ m Optical Window Increased Depletion Widths, Decreased Electron Velocity 38 GHz in InGaAs Normalized Frequency Response Bias Dependent Bandwidths J. H. Jang et. al. Photon. Technol. Lett., 2001 pp. 1097-1099 Advanced Processing and Circuits Group
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